A wireless communication system has been widely developed to provide various kinds of communication services such as voice and data. Generally, the wireless communication system is a multiple access system that can support communication with multiple users by sharing available system resources (bandwidth, transmission power, etc.). Examples of the multiple access system include a code division multiple access (CDMA) system, a frequency division multiple access (FDMA) system, a time division multiple access (TDMA) system, an orthogonal frequency division multiple access (OFDMA) system, a single carrier frequency division multiple access (SC-FDMA) system, and a multi carrier-frequency division multiple access (MC-FDMA) system.
If a wireless communication system transmits a data unit (e.g., packet), a receiver should notify the success or failure of the data unit reception to a transmitter. If the data unit reception is successful, an ACK (acknowledgement) is transmitted, so that the transmitter can transmit a new data unit, and if the data unit reception is unsuccessful (or failed), a NACK (Negative-ACK) is transmitted, so that the transmitter retransmits the corresponding data unit. Such operation is referred to as an ARQ (Automatic Repeat reQuest). An HARQ (hybrid ARQ) is a method consisting of a combination of the ARQ and channel coding. The HARQ may combine a retransmitted data unit with an already-received data unit, thereby reducing an error rate. In the HARQ, the ACK/NACK (A/N) is transmitted by using a physical channel signaling method. The method for realizing the HARQ may broadly include a Chase Combining (CC) method and an Incremental Redundancy (IR) method.